The average wait time is determined by averaging the wait time for your role in that queue for the last hour across the data center. When you see the time start increasing like that its either because you hit that time where people are logging off for the night so queues are naturally taking longer or because you're queueing for something without much historical data to give an accurate prediction. In the later case it usually starts at 5min and keeps incrementing by a few minutes each time you pass it; in effect you're creating the base for the average wait time. You'll notice this a lot if you log in right after maint and queue for something.